Overlay measuring pattern, photomask, and overlay measuring method and apparatus

ABSTRACT

An overlay measuring pattern has a rectangular portion and band-shaped portions separated from opposite sides of the rectangular portion by an equal distance and parallel to the sides. Even when a photoresist shrinks upon heating after the overlay measuring pattern is transferred to the photoresist, the taper amounts of the sides of the rectangular portion transferred to the photoresist equal each other in their sections between opposite sides. Hence, the overlay shift can be accurately obtained.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to measurement of overlay betweenthe patterns of lower and upper layers in manufacturing a semiconductordevice.

[0003] 2. Description of the Related Art

[0004] In manufacturing a semiconductor device, processes of forming afilm to be etched, forming a photoresist, transferring a pattern from aphotomask to the photoresist by photolithography, transferring thepattern from the photoresist to the film to be etched by etching, andremoving the photoresist are repeated a plurality of number of times toform a circuit pattern having a plurality of layers or the like. Hence,an overlay shift between the photomasks is an important factor indetermining the degree of integration of the semiconductor device.

[0005] To cope with this overlay shift, an overlay measuring pattern isformed in, e.g., a scribing region of a photomask, where neither circuitpattern nor the like necessary for the manufacture of a semiconductordevice is formed, and the overlay measuring pattern is measured by anoverlay measuring apparatus of an optical pattern edge detection scheme,thereby measuring the overlay accuracy between the photomasks.

[0006]FIGS. 1A and 1B show a popular related art of an overlay measuringpattern which is called a box pattern. As shown in FIG. 1A, this relatedart comprises an overlay measuring pattern 11 called a main scale and anoverlay measuring pattern 12 called a sub scale. The overlay measuringpatterns 11 and 12 are formed on photomasks for forming the patterns oflower and upper layers, respectively.

[0007] The overlay measuring pattern 11 has a rectangular portion 13 asan opening in the pattern formed from chromium or the like. The overlaymeasuring pattern 12 has a rectangular portion 14 as a pattern formedfrom chromium or the like. The overlay measuring patterns 11 and 12 havecharacters such as “No. 1” and “No. 2” to identify the target transferfilms on which the patterns are transferred, as shown in FIG. 1A. Thesecharacters are also transferred upon transferring the patterns.

[0008] The definitions of main and sub scales are relative. The overlaymeasuring pattern 11 may be used as the sub scale, and the overlaymeasuring pattern 12 may be used as the main scale. The rectangularportion 13 may be a pattern formed from chromium or the like, and therectangular portion 14 may be an opening in the pattern formed fromchromium or the like. Alternatively, both of the rectangular portions 13and 14 may be patterns formed from chromium or the like, or both of therectangular portions 13 and 14 may be openings.

[0009] The overlay measuring patterns 11 and 12 are formed on thephotomasks such that the centers of the rectangular portions 13 and 14match when the photomasks are accurately overlaid, as shown in FIG. 1B.Hence, when the overlay measuring patterns 11 and 12 are overlaid, thedifference between a distance x₁ from a left side X₁ of the rectangularportion 13 to the left side X₁ of the rectangular portion 14 and adistance x₂ from a right side X₂ of the rectangular portion 13 to theright side X₂ of the rectangular portion 14, (x₁−x₂)/2, represents theoverlay shift in the X direction, i.e., in the horizontal direction.

[0010] Similarly, when the overlay measuring patterns 11 and 12 areoverlaid, the difference between a distance y₁ from a lower side Y₁ ofthe rectangular portion 13 to the lower side Y₁ of the rectangularportion 14 and a distance y₂ from an upper side Y₂ of the rectangularportion 13 to the upper side Y₂ of the rectangular portion 14,(y₁−y₂)/2, represents the overlay shift in the Y direction, i.e., in thevertical direction.

[0011] When patterns are to be transferred to three or more targettransfer films, two or more overlay measuring patterns 11 as main scalesare consecutively formed on the target transfer film as the first layer,as shown in FIG. 2. Sub scales corresponding to the second andsubsequent layers are overlaid on the respective overlay measuringpatterns 11. Using the overlay measuring pattern 11 transferred to thetarget transfer film as the first layer as a reference, the overlayaccuracy of the photomasks of the second and subsequent layers ismeasured.

[0012] A photoresist is exposed and developed, and then, heated to atemperature within the range of about 90° C. to 150° C. by a hot plateor oven for drying and hardening. The upper portion of the photoresistdefining the opening end largely thermally shrinks due to surfacetension, so the photoresist has a tapered sectional shape. Since thetaper amount depends on the volume of a photoresist, the pater amount ofa photoresist applied to the same thickness is proportional to its area.

[0013] As shown in FIG. 3A, of areas S₁ to S₄ between the sides of therectangular portion 13 of the overlay measuring pattern 11 transferredto a positive photoresist 15 and the edges of the overlay measuringpattern 11, the area S₂ of a portion in contact with the right side ofthe rectangular portion 13 and the area S₄ of a portion in contact withthe left side may be equal. However, since characters “No. 1” need bewritten at a portion in contact with the upper side of the rectangularportion 13, the area S₁ of the portion in contact with the upper side islarger than the area S₃ of a portion in contact with the lower side.

[0014] For this reason, even when taper amounts Δx₁ and Δx₂ of thephotoresist 15 have a relation Δx₁=Δx₂ in an X-direction section, asshown in FIG. 3B, taper amounts Δy₁ and Δy₂ of the photoresist 15 have arelation Δy₁<Δy₂ in a Y-direction section, as shown in FIG. 3C.

[0015] For example, when two overlay measuring patterns 11 areconsecutively formed in the X direction, as shown in FIG. 4A, the taperamounts Δy₁ and Δy₂ of the photoresist 15 have a relation Δy₁<Δy₂ in aY-direction section, as shown in FIG. 4C, and at the same time, thetaper amounts Δx₁ and Δx₂ of the photoresist 15 also have a relationΔx₁<Δx₂ in an X-direction, as shown in FIG. 4B.

[0016] When the taper amount of the sectional shape of the photoresist15 on a film 16 to be etched is nonuniform, as shown FIG. 5A, theretreat amount of the etched film 16 increases at the largely taperedportion of the photoresist 15, as shown in FIG. 5B. For this reason, asshown in FIG. 5C, the overlay measuring pattern 11 is transferred to theetched film 16 while the positions of sides of the rectangular portion13 shift from the original transfer positions.

[0017] Consequently, when the next film 17 to be etched is formed on theetched film 16, and the rectangular portion 14 of the overlay measuringpattern 12 is transferred to a photoresist 18 on the film 17 to beetched, as shown in FIG. 6A, a measurement error occurs as if therectangular portion 14 shifted in both the X and Y directions, althoughthe rectangular portion 14 is accurately formed at the originalposition, as can be seen from FIG. 6B.

[0018] Reportedly, when a 1.2-μm thick photoresist is heated at 120° C.for 90 sec, the above-described measurement error reaches 0.1 μm. Hence,erroneous information may be fed back to the next lot, or thephotoresist 18 is reformed, resulting a decrease in yield.

SUMMARY OF THE INVENTION

[0019] It is an object of the present invention to provide an overlaymeasuring pattern, a photomask, and an overlay measuring method andapparatus capable of accurately measuring overlay shift between anoverlay measuring pattern used for transfer to a lower target transferfilm and an overlay measuring pattern used for transfer to an uppertarget transfer film.

[0020] An overlay measuring pattern according to the present inventionhas band-shaped portions separated from opposite sides of a rectangularportion by an equal distance and parallel to these sides of therectangular portion. Hence, the areas between sides and band-shapedportions equal each other for portions of an identical length at theopposite sides of the rectangular portion.

[0021] For this reason, even when a photoresist is heated after theoverlay measuring pattern is transferred to the photoresist, and thephotoresist shrinks due to the surface tension of the photoresist, thetaper amounts of the sides of the rectangular portion transferred to thephotoresist equal each other in their sections between the oppositesides.

[0022] Consequently, even when the overlay measuring pattern istransferred to a film to be etched by etching using the photoresist as amask, and the sides of the rectangular portion transferred to thephotoresist shift from those of the rectangular portion transferred tothe etched film, the shifts equal each other between the opposite sidesof the rectangular portion transferred to the etched film.

[0023] That is, even when pattern size shifts are generated between therectangular portion of the overlay measuring pattern and the rectangularportion transferred to the target transfer film, the pattern size shiftsequal each other between the opposite sides of the rectangular portiontransferred to the target transfer film.

[0024] When a rectangular portion included within or including therectangular portion transferred to the target transfer film as a lowerlayer when viewed from above is transferred to a target transfer film asan upper layer, and the distance between a side of the rectangularportion transferred to the target transfer film as the lower layer and aside of the rectangular portion transferred to the target transfer filmas the upper layer, which sides oppose each other, is measured, anyoverlay shift between the overlay measuring pattern used for transfer tothe target transfer film as the lower layer and that used for transferto the target transfer film as the upper layer can be accuratelymeasured.

[0025] In a preferred overlay measuring pattern according to the presentinvention, since the ratio of the length of the band-shaped portionopposing a side of the rectangular portion to the length of the side is50% or more, the ratio of the length of a portion of the side, whichopposes the band-shaped portion, to the length of the side is high. Forthis reason, even when pattern size shifts are generated between therectangular portion of the overlay measuring pattern and the rectangularportion transferred to the target transfer film, those portions whichhave the same pattern size shifts are long between the opposite sides ofthe rectangular portion transferred to the target transfer film.

[0026] Therefore, any overlay shift between the overlay measuringpattern used for transfer to the target transfer film as the lower layerand the overlay measuring pattern used for transfer to the targettransfer film as the upper layer can be accurately and easily measured.

[0027] A photomask according to the present invention comprises anoverlay measuring pattern having a rectangular portion and band-shapedportions separated from opposite sides of the rectangular portion by anequal distance and parallel to these sides. Hence, the areas betweensides and band-shaped portions equal each other for portions of anidentical length at the opposite sides of the rectangular portion.

[0028] For this reason, even when pattern size shifts are generatedbetween the rectangular portion of the overlay measuring pattern in thephotomask and the rectangular portion transferred to the target transferfilm, the pattern size shifts equal each other between the oppositesides of the rectangular portion transferred to the target transferfilm.

[0029] When a rectangular portion included within or including therectangular portion transferred to the target transfer film as a lowerlayer when viewed from above is transferred to a target transfer film asan upper layer, and the distance between a side of the rectangularportion transferred to the target transfer film as the lower layer and aside of the rectangular portion transferred to the target transfer filmas the upper layer, which sides oppose each other, is measured, anyoverlay shift between the photomask used for transfer to the targettransfer film as the lower layer and that used for transfer to thetarget transfer film as the upper layer can be accurately measured.

[0030] In a preferred photomask according to the present invention,since the ratio of the length of the band-shaped portion in the overlaymeasuring pattern, which opposes a side of the rectangular portion, tothe length of the side is 50% or more, the ratio of the length of aportion of the side, which opposes the band-shaped portion, to thelength of the side is high.

[0031] For this reason, even when pattern size shifts are generatedbetween the rectangular portion of the overlay measuring pattern in thephotomask and the rectangular portion transferred to the target transferfilm, those portions which have the same pattern size shifts are longbetween the opposite sides of the rectangular portions transferred tothe target transfer film. Therefore, any overlay shift between thephotomask used for transfer to the target transfer film as the lowerlayer and the photomask used for transfer to the target transfer film asthe upper layer can be accurately and easily measured.

[0032] An overlay measuring method according to the present inventionuses first and second patterns each having a rectangular portion andband-shaped portions separated from opposite sides of the rectangularportion by an equal distance and parallel to the sides of therectangular portion. In both the first and second patterns, the areasbetween sides and band-shaped portions equal each other for portions ofan identical length at the opposite sides of the rectangular portion.

[0033] For this reason, even when pattern size shifts are generatedbetween the rectangular portion of the first pattern and the rectangularportion transferred to the target transfer film as the lower layer, andpattern size shifts are generated between the rectangular portion of thesecond pattern and the rectangular portion transferred to the targettransfer film as the upper layer, the pattern size shifts equal eachother between the opposite sides of the rectangular portions transferredto the target transfer films as the lower and upper layers.

[0034] The rectangular portions of the first and second patterns havesizes with which one includes the other, and the second pattern istransferred to the target transfer film as the upper layer whileoverlaying the rectangular portion of the second pattern on therectangular portion transferred to the target transfer film as the lowerlayer. The distance between a side of the rectangular portiontransferred to the target transfer film as the lower layer and a side ofthe rectangular portion transferred to the target transfer film as theupper layer, which sides oppose each other, corresponds to the overlayshift between the first and second patterns.

[0035] When this distance is measured, the overlay shift between thefirst pattern used for transfer to the target transfer film as the lowerlayer and the second pattern used for transfer to the target transferfilm as the upper layer can be accurately measured on the basis ofdistances in the same directions.

[0036] In a preferred overlay measuring method according to the presentinvention, in both the first and second patterns, since the ratio of thelength of the band-shaped portion in the overlay measuring pattern,which opposes a side of the rectangular portion, to the length of theside is 50% or more, the ratio of the length of a portion of the side,which opposes the band-shaped portion, to the length of the side ishigh.

[0037] For this reason, even when pattern size shifts are generatedbetween the rectangular portion of the first pattern and the rectangularportion transferred to the target transfer film as the lower layer andbetween the rectangular portion of the second pattern and therectangular portion transferred to the target transfer film as the upperlayer, those portions which have the same pattern size shifts are longbetween the opposite sides of the rectangular portions transferred tothe target transfer films of the lower and upper layers.

[0038] As a consequence, any overlay shift between the first and secondpatterns can be easily obtained from the distance between opposite sidesof the rectangular portions transferred to the target transfer films asthe lower and upper layers. Therefore, any overlay shift between thefirst pattern used for transfer to the target transfer film as the lowerlayer and the second pattern used for transfer to the target transferfilm as the upper layer can be accurately and easily measured.

[0039] In an overlay measuring apparatus according to the presentinvention, first and second patterns which overlap each other and eachof which has a rectangular portion and band-shaped portions separatedfrom opposite sides of the rectangular portion by an equal distance andparallel to the sides of the rectangular portion, one rectangularportion including the other are scanned to obtain edge signals from theedges of the patterns. To obtain the distance between opposite sides ofthe first and second patterns, only edge signals corresponding to thesides are used.

[0040] For this reason, although band-shaped portions are formed in thefirst and second patterns, edge signals obtained from these band-shapedportions are neglected, and the distance between opposite sides of thefirst and second patterns can be obtained. Hence, any overlay shiftbetween the first and second patterns can be accurately measured.

BRIEF DESCRIPTION OF THE DRAWINGS

[0041]FIGS. 1A and 1B are views showing a related art of the presentinvention, in which FIG. 1A is a plan view of an overlay measuringpattern as a main scale and an overlay measuring pattern as a sub scale,and FIG. 1B is a plan view showing a state wherein the overlay measuringpattern as the main scale and the overlay measuring pattern as the subscale are overlaid;

[0042]FIG. 2 is a plan view showing a state wherein four patterns of therelated art of the present invention are consecutively formed in the Xdirection;

[0043]FIGS. 3A to 3C are views showing a state wherein one pattern ofthe related art is formed, in which FIG. 3A is a plan view, FIG. 3B is alongitudinal sectional view taken along a line B-B in FIG. 3A and FIG.3C is a longitudinal sectional view taken along a line C-C in FIG. 3A;

[0044]FIGS. 4A to 4C are views showing a state wherein two patterns ofthe related art are consecutively formed in the X direction, in whichFIG. 4A is a plan view, FIG. 4B is a longitudinal sectional view takenalong a line B-B in FIG. 4A and FIG. 4C is a longitudinal sectional viewtaken along a line C-C in FIG. 4A;

[0045]FIGS. 5A to 5C are views showing a state wherein the overlaymeasuring pattern of the related art is transferred, in which FIG. 5A isa longitudinal sectional view showing a state wherein the overlaymeasuring pattern is transferred to a photoresist, FIG. 5B is alongitudinal sectional view showing a state wherein the overlaymeasuring pattern is transferred to an etched film and FIG. 5C is a planview showing a state wherein the overlay measuring patterns aretransferred to the etched film;

[0046]FIGS. 6A and 6B are views showing a state wherein the overlaymeasuring pattern as the main scale and the overlay measuring pattern asthe sub scale of the related art are overlaid, in which FIG. 6A is alongitudinal sectional view taken along a line A-A in FIG. 6B, and FIG.6B is a plan view;

[0047]FIGS. 7A and 7B are views showing the first embodiment of thepresent invention, in which FIG. 7A is a plan view of an overlaymeasuring pattern as a main scale and an overlay measuring pattern as asub scale, and FIG. 7B is a plan view showing a state wherein theoverlay measuring pattern as the main scale and the overlay measuringpattern as the sub scale are overlaid;

[0048]FIGS. 8A to 8C are views showing a state wherein two patterns ofthe first embodiment are consecutively formed in the X direction, inwhich FIG. 8A is a plan view, FIG. 8B is a longitudinal sectional viewtaken along a line B-B in FIG. 8A and FIG. 8C is a longitudinalsectional view taken along a line C-C in FIG. 8A;

[0049]FIG. 9 is a plan view showing a state wherein two patterns of thesecond embodiment of the present invention are consecutively formed inthe X direction; and

[0050]FIG. 10 is a plan view showing a state wherein two patterns of thethird embodiment of the present invention are consecutively formed inthe X direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051] The first to third embodiments of the present invention will bedescribed below with reference to FIGS. 7A to 10. FIGS. 7A to 8C showthe first embodiment. As shown in FIG. 7A, the first embodimentcomprises an overlay measuring pattern 21 as a main scale and an overlaymeasuring pattern 22 as a sub scale. The overlay measuring patterns 21and 22 are formed on photomasks for forming lower and upper patterns,respectively.

[0052] The overlay measuring pattern 21 has a rectangular portion 23 asan opening in the pattern formed from chromium or the like. The overlaymeasuring pattern 22 has a rectangular portion 24 as a pattern formedfrom chromium or the like. The overlay measuring patterns 21 and 22 havecharacters such as “No. 1” and “No. 2” to identify the target transferfilms on which the patterns are transferred, as shown in FIG. 7A. Thesecharacters are also transferred upon transferring the patterns.

[0053] The length of lower and upper sides Y₁ and Y₂ of the rectangularportion 23 is represented by i, and the length of left and right sidesX₁ and X₂ is represented by j. The length of the lower and upper sidesY₁ and Y₂ of the rectangular portion 24 is represented by o, and thelength of the left and right sides X₁ and X₂ is represented by p. Thesedefinitions are substantially the same as in the related art shown inFIGS. 1A and 1B.

[0054] In the first embodiment, however, the overlay measuring pattern21 has band-shaped portions 25 a and 25 b as openings which areseparated from the left and right sides X₁ and X₂ of the rectangularportion 23 by equal distances b and c and are parallel to the left andright sides X₁ and X₂, respectively, and band-shaped portions 25 c and25 d as openings which are separated from the lower and upper sides Y₁and Y₂ by equal distances f and g and are parallel to the lower andupper sides Y₁ and Y₂, respectively.

[0055] Widths a, d, e and h of the band-shaped portions 25 a to 25 d canbe arbitrarily set within the range in which the band-shaped portionscan be accommodated in the overlay measuring pattern 21 as far as theyhave values equal to or larger than the resolution limit ofphotolithography. However, lengths n and l of the band-shaped portions25 a and 25 b are preferably 50% or more of the length j of the left andright sides X₁ and X₂, respectively, and lengths k and m of theband-shaped portions 25 c and 25 d are preferably 50% or more of thelength i of the lower and upper sides Y₁ and Y₂, respectively.

[0056] In the first embodiment, the overlay measuring pattern 22 alsohas band-shaped portions 26 a to 26 d. The relationship between theband-shaped portions 26 a to 26 d and the rectangular portion 24 is thesame as the relationship between the band-shaped portions 25 a to 25 dand the rectangular portion 23 of the overlay measuring pattern 21.

[0057] In the first embodiment, in, e.g., the overlay measuring pattern21, for portions with the same length at the left and right sides X₁ andX₂ and portions with the same length at the lower and upper sides Y₁ andY₂, the area between the left side X₁ and band-shaped portion 25 aequals the area between the right side X₂ and band-shaped portion 25 bwhile the area between the lower side Y₁ and band-shaped portion 25 cequals the area between the upper side Y₂ and band-shaped portion 25 d.These relationships also apply to the overlay measuring pattern 22.

[0058] For this reason, even when the photoresist thermally shrinks dueto surface tension after it is exposed and developed using the overlaymeasuring patterns 21 and 22 and then heated for drying and hardening,the taper amounts of the rectangular portions 23 and 24 transferred tothe photoresist equal between the left and right sides X₁ and X₂ andbetween the lower and upper sides Y₁ and Y₂ in their sections.

[0059] Consequently, when the overlay measuring patterns 21 and 22 aretransferred to an etched film by etching using the photoresist as amask, and the sides X₁, X₂, Y₁ and Y₂ of the rectangular portions 23 and24 transferred to the etched film retreat from the sides X₁, X₂, Y₁ andY₂ of the rectangular portions 23 and 24 transferred to the photoresist,the retreat amounts equal each other between the left and right sides X₁and X₂ of the rectangular portions 23 and 24 transferred to the etchedfilm and also between the lower and upper sides Y₁ and Y₂.

[0060] Hence, as shown in FIG. 7B, when the overlay measuring pattern 21is transferred to an etched film 16 and the overlay measuring pattern 22is transferred to a photoresist 18, as in FIG. 6A, the differencebetween a distance x₁ from the left side X₁ of the rectangular portion23 to the left side X₁ of the rectangular portion 24 and a distance x₂from the right side X₂ of the rectangular portion 23 to the right sideX₂ of the rectangular portion 24, (x₁−x₂)/2, accurately represents theoverlay shift in the X direction.

[0061] Similarly, the difference between a distance y₁ from the lowerside Y₁ of the rectangular portion 23 to the lower side Y₁ of therectangular portion 24 and a distance y₂ from the upper side Y₂ of therectangular portion 23 to the upper side Y₂ of the rectangular portion24, (y₁−y₂)/2, accurately represents the overlay shift in the Ydirection.

[0062]FIGS. 8A to 8C show a state wherein two overlay measuring patterns21 are consecutively formed on a photoresist 15 in the X direction, asin FIGS. 4A to 4C. In this case as well, taper amounts Δx₁ and Δx₂ ofthe X-direction section of the photoresist 15 have a relation Δx₁=Δx₂,and taper amounts Δy₁ and Δy₂ of the Y-direction section of thephotoresist 15 also have a relation Δy₁=Δy₂, so overlay shifts in the Xand Y directions can be accurately obtained.

[0063] In the related art shown in FIGS. 1A and 1B, as is apparent fromFIG. 1B, when the overlay measuring patterns 11 and 12 are scanned inthe X direction, edge signals are sequentially generated from the sideX₁ of the rectangular portion 13, the side X₁ of the rectangular portion14, the side X₂ of the rectangular portion 14 and the side X₂ of therectangular portion 13, and the distances x₁ and x₂ can be easilyobtained from these edge signals. The distances y₁ and y₂ can also beeasily obtained.

[0064] In the first embodiment, however, as is apparent from FIG. 7B,when the overlay measuring patterns 21 and 22 are scanned in the Xdirection, edge signals are generated not only from the sides X₁ and X₂of the rectangular portions 23 and 24 but also from the band-shapedportions 25 a, 25 b, 26 a and 26 b. For this reason, to obtain thedistances x₁ and x₂, the arithmetic section of the overlay measuringapparatus uses only the edge signals generated from the sides X₁ and X₂of the rectangular portions 23 and 24 and neglects the edge signalsgenerated from the band-shaped portions 25 a, 25 b, 26 a and 26 b.

[0065] This also applies to Y direction scanning of the overlaymeasuring patterns 21 and 22. That is, to obtain the distances y₁ andy₂, the arithmetic section of the overlay measuring apparatus uses onlythe edge signals generated from the sides Y₁ and Y₂ of the rectangularportions 23 and 24 and neglects the edge signals generated from theband-shaped portions 25 c, 25 d, 26 c and 26 d.

[0066]FIG. 9 shows an overlay measuring pattern 21 as a main scale ofthe second embodiment. The overlay measuring pattern 21 of the secondembodiment substantially has the same arrangement as that of the firstembodiment shown in FIGS. 7A to 8C except that band-shaped portions 25 ato 25 d are connected around a rectangular portion 23 because theband-shaped portions 25 a and 25 b are longer than left and right sidesX₁ and X₂ and the band-shaped portions 25 c and 25 d are longer thanlower and upper sides Y₁ and Y₂.

[0067] Although not illustrated, an overlay measuring pattern as a subscale of the second embodiment has the same arrangement as that of theoverlay measuring pattern as the main scale. In the second embodiment aswell, the same effect as that of the first embodiment can be obtained.

[0068]FIG. 10 shows an overlay measuring pattern 21 as a main scale ofthe third embodiment. The overlay measuring pattern 21 of the thirdembodiment substantially has the same arrangement as that of the secondembodiment shown in FIG. 9 except that band-shaped portions 25 a to 25 dbecome wider to extend to the edges of the overlay measuring pattern 21.

[0069] Although not illustrated, an overlay measuring pattern as a subscale of the third embodiment has the same arrangement as that of theoverlay measuring pattern as the main scale. In the third embodiment aswell, the same effect as that of the first and second embodiments can beobtained.

What is claimed is:
 1. An overlay measuring pattern comprising: arectangular portion; and band-shaped portions separated from oppositesides of said rectangular portion by an equal distance and parallel tothe sides.
 2. A pattern as claimed in claim 1 , wherein a ratio of alength of said band-shaped portion opposing the side to a length of theside is not less than 50%.
 3. A photomask comprising an overlaymeasuring pattern having a rectangular portion and band-shaped portionsseparated from opposite sides of said rectangular portion by an equaldistance and parallel to the sides.
 4. A photomask as claimed in claim 3, wherein a ratio of a length of said band-shaped portion opposing theside to a length of the side is not less than 50%.
 5. An overlaymeasuring method comprising the steps of: transferring a first patternhaving a first rectangular portion and band-shaped portions separatedfrom opposite sides of said first rectangular portion by an equaldistance and parallel to the sides to a target transfer film as a lowerlayer; transferring a second pattern having a second rectangular portionand band-shaped portions separated from opposite sides of said secondrectangular portion by an equal distance, parallel to the sides andhaving a size with which one of said first and second rectangularportions includes the other to a target transfer film as an upper layerwhile overlaying said second rectangular portion on said firstrectangular portion; measuring distances between sides of said firstrectangular portion transferred to the target transfer film as the lowerlayer and sides of said second rectangular portion, which oppose thesides of said first rectangular portion, transferred to the targettransfer film as the upper layer; and obtaining an overlay shift betweensaid first and second patterns on the basis of the distances in the samedirection.
 6. A method as claimed in claim 5 , wherein a ratio of alength of said band-shaped portion opposing the side to a length of theside is not less than 50%.
 7. An overlay measuring apparatus comprising:an arithmetic section for obtaining a distance between opposite sides offirst and second patterns using, of edge signals obtained from edges ofsaid patterns upon scanning said first and second patterns overlaid eachother, only edge signals corresponding to the sides, each of said firstand second patterns having a rectangular portion and band-shapedportions separated from opposite sides of said rectangular portion by anequal distance and parallel to the sides, and said rectangular portionshaving sizes with which one of said rectangular portions includes theother.